A new race of the downy mildew pathogen (Peronospora farinosa f. sp. spinaciae = P. effusa) on spinach was first identified in March of 2015 in Salinas, CA, U.S. This race was able to overcome most of the race 1-15 varieties evaluated. The isolate, originally designated UA201519B, was characterized based on disease development on a standard set of differential varieties. Subsequently, isolates with the same reaction pattern on the differential set have been found in numerous locations. After careful evaluation of the significance of this development to the spinach industry, the International Working Group on Peronospora (IWGP) has denominated isolate UA201519B race Pfs: 16.

Race Pfs: 16 is able to infect the differentials Viroflay, Resistoflay, Clermont, Lazio, Pigeon, and Meerkat, but not able to infect Califlay, Campania, Boeing (Avenger), Lion, Whale, and Caladonia.

The IWGP is continuously monitoring the appearance of strains of the pathogen that deviate in virulence from the known races. In this way the IWGP aims to promote a consistent and clear communication between public and private entities, such as the seed industry, growers, scientists, and other interested parties about all resistance-breaking races that are persistent enough to survive over several years, occur in a wide area, and cause a significant economic impact.

The IWGP is located in The Netherlands and is administered by Plantum NL. The IWGP consists of spinach seed company representatives (Pop Vriend, Monsanto, RijkZwaan, Bayer, Takii, Sakata, Bejo, Enza, Syngenta, and Advanseed) and Naktuinbouw, and is supported by research centers at the University of Arkansas and the University of California Cooperative Extension (Monterey County) in the U.S. Researchers all over the world are invited to join the IWGP initiative and use the common host differential set to identify new isolates.

For more information on this subject you can contact Jim Correll (jcorrell@uark.edu), Steve Koike (stkoike@ucdavis.edu), Diederik Smilde (d.smilde@naktuinbouw.nl), or the IWGP chairperson Jan de Visser (JandeVisser@popvriendseeds.nl).

Disease reactions of race 16 (UA201519B) observed on spinach differentials by the IWGP compiled February 2016.

^aDisease reactions observed in controlled inoculation tests

by the participants of the IWGP. “+” indicates

susceptible; “-“ indicates resistant disease reaction.

CropManage 2.0 Workshop: Hands-on training

Monterey County Agricultural Center Conference Room

 1432 Abbott St, Salinas CA 93901

Thursday, March 31 2016

(8:30 am – 12 pm)

We will offer a hands-on training to learn in depth about the features of CropManage 2.0, a free online decision support tool for water and nutrient management of coastal crops.  CropManage was recently revised with a new user interface and additional capabilities, and now supports iceberg and romaine lettuce, spinach, celery, broccoli, cauliflower, cabbage, and strawberries.  

Considering that the drought may continue into a 5th year, and water quality regulations continue to be linked to nutrient management, efficiently using water and nitrogen fertilizer is a high priority for Central Coast growers. CropManage can be a useful tool for providing quick decision support on water and nutrient management on a field-by-field basis. This training will provide an opportunity to learn how to use CropManage for improving the efficiency of your farming operations or for adding value to your consulting services. 

The workshop will take you step-by-step on how to navigate and use CropManage for assisting with fertilizer and water management decisions and record keeping. Wi-Fi internet access is available at our conference room so please bring a laptop or tablet computer so that you can follow along as we tour through the features of the software.   There should be sufficient time to answer questions as we cover the following topics: 

Agenda:

8:30 – 9:00      Registration and Refreshments

9:00 – 9:30      Introduction and update on CropManage 2.0

9:30-10:15      Getting started with CropManage

10:15  (Break)

10:30 – 11:15 Strategies for using CropManage for decision support and record keeping

11:15- 11:45  Advanced features and interfacing sensors with CropManage

11:45-12:00   Discussion of new features or changes needed

Certified Crop Adviser CEU hours (1 hr soil and water management, 1 hr nutrient management, 0.5 hrs professional development)

To keep the group size manageable so that we can provide individual help, we would like to limit the workshop to 30 participants.  If you have attended previous workshops and or feel proficient in using the on-line tool, then you are welcome to just attend the second half of the workshop (10:30-12 pm).  Whether or not you plan to attend the entire or part of the workshop, please RSVP in advance by sending an email to larriaga@ucanr.edu with the subject heading “CropManage workshop” and let us know the number of participants in your group.  We will email you a confirmation. Alternatively, you can contact our office at 831-759-7370.  Thank you, and I hope to see you soon.

Michael Cahn

Please feel free to contact us if you need special accommodations 

Summary: The ability of broccoli to scavenge N from depths below 2 feet was evaluated in 2014 and 2015. Broccoli roots extend down to three feet in the soil by the end of the crop cycle. Broccoli is capable of scavenging nitrogen from the 2^nd and 3^rd foot in the soil, but adequate nitrogen is required to help develop the crop at an early stage for broccoli to achieve maximum yield. Most scavenging of nitrate deeper in the soil profile occurs in the final third of the crop cycle. In a survey of 10 broccoli fields following a prior lettuce crop broccoli took up >80% of the residual + fertilizer N in 4 of these fields. If residual soil nitrate levels are not too high, broccoli can take up a high percent of the N mineralized from the residue of the previous lettuce crop and reduce levels of soil nitrate that is at risk of leaching. Broccoli residue also mineralizes rapidly and releases large quantities of nitrate that could provide for the N needs of a subsequent crop.

Background:Summer-grown broccoli and other cole crops such as cauliflower and cabbage have been shown to routinely take up more nitrogen than is typically applied as fertilizer. This indicates that these crops scavenge excess nitrogen from the soil profile. They are deep rooted and utilize soil nitrate in the second and even the third foot of soil. The phenomenon of scavenging is not unique to the cole crops, as we have observed scavenging in some fields of peppers, celery and lettuce. The question is, however: Where does the residual soil nitrate that these crops scavenge come from?

Residual levels of soil nitrates comes from several sources: mineralization of soil organic matter and prior crop residues, unused N from fertilizers applied to prior crops and nitrate in irrigation water. In warm summer soils, nitrate is the main form of mineral N in the soil and typically ranges from 5 to 30 ppm nitrate-N. Soil ammonium-N levels in the soil in the summer are much less and typically range from 1 to 3 ppm. The nitrate molecule has a negative charge and is not adsorbed onto soil colloids, but rather resides in moisture held in soil pores. As such, nitrate is at risk to be transported below the rooting zone of crops with excess water from irrigation or rainfall. It is therefore best to measure soil nitrate immediately prior to making fertilizer decisions. For this reason, the nitrate quick test is the most common method of measuring soil nitrates in our cool season vegetable area because it allows growers to get the results of soil nitrate levels quickly in order to make fertilizer application decisions.

Residual soil nitrate levels must be high enough to effectively supply a fast growing cool season vegetable. A sufficiency threshold of 20 ppm nitrate-N was first proposed for cool season vegetable crops by Tim Hartz, which if measured in the first foot of soil, is equivalent to 76 lbs N/acre. This quantity of nitrogen is sufficient to supply the immediate needs of the vegetable crop for two to three weeks.

Nitrogen Scavenging Studies: Uptake of N from various depths in the profile. In 2014-15 we evaluated fertilization strategies of broccoli and its ability to scavenge residual soil nitrate from the soil following a lettuce crop. Broccoli roots were found to reach three feet deep in the soil by the end of the cropping cycle (Figure 1). To evaluate the ability of broccoli to utilize nitrogen from deeper in the soil profile, we conducted a trial at the USDA Spence Research Station in which we applied nitrogen using drip tape placed on the surface, and at depths of 12, 18 and 24 inches using equipment for injecting drip tape below the soil surface. The soil profile was low in residual soil nitrate at the beginning of the trial and the same quantity of N (150 lbs N/A) was applied to all the treatments. Fertilizer treatments were applied 27, 38 and 48 days after the first germination water.

The yield was lower in the treatments where the fertilizer was applied at 12 & 18 inch deep in the soil and was particularly low where it was applied 24 inches deep (data not shown). Because it takes time for broccoli roots to grow down through the soil profile, it appears from these results that the early growth suffered from lack of nitrogen, since the root system was not adequately developed to take up the fertilizer from deeper in the soil profile. These results suggest that broccoli needs an adequate supply of N in the upper foot during the early stages of development, even when an ample N may be available deeper in the soil.

Uptake of residual soil N by broccoli: We also studied the ability of broccoli to take up residual soil nitrate left over from the prior lettuce crop. We surveyed a total of 10 fields over two years, evaluating the amount of residual soil nitrate at the beginning of the broccoli cropping cycle. Nitrogen was measured in the top three feet to account for the total amount of nitrogen accessible to the crop. The levels of soil nitrate in the top three feet of soil at the beginning of the broccoli cropping cycle ranged from 134 to 431 lbs N/A (Table 1). This wide variation in the range may have to do with fertilizer practices in the prior lettuce crop and N mineralized from crop residues. Fertilizer applications to the broccoli crops ranged from 169 to 240 lbs N/A, and the ratio of applied fertilizer N to crop N uptake ranged from 0.48 to 1.09.

The percent of initial mineral N taken up by the broccoli crop ranged from 40 to 106%, with four out of the ten sites having N uptake by the broccoli crop of greater than 82% uptake of initial residual soil nitrate + fertilizer N. The amount of residual soil nitrate left in the top three feet following the broccoli crop ranged from 9 to 192 lbs N/A. The results from this sampling of broccoli fields show that, under the right conditions, broccoli can take up a large percentage of the residual soil nitrogen left by a previous lettuce crop. However, if residual soil nitrate levels are too high, broccoli will not be able to absorb a large percent of the initial load.

In Table 1, the last column shows the levels of nitrate unaccounted-for. This estimate probably reflects leached nitrate. In all sites with unaccounted nitrate levels of >100 lbs N/A, the total amount of mineral N available for the crop (initial residual nitrate + fertilizer) was >440 lbs N/A. The ratio of crop water applied to crop ETc ranged from 1.0 to 2.1, but sites with the highest potential leaching did not necessarily correlate with the more efficient irrigation ratios.

On-farm evaluation of N uptake from 2^nd foot of soil: We conducted an on-farm evaluation to see if 2^nd-foot soil samples could be used to guide fertilizer application decisions. This trial was conducted to refine the algorithms used in CropManage (CM) to guide water and fertilizer decisions for broccoli. Table 2 shows the quantities of fertilizer applied to the CM and standard treatments. The quantities of fertilizer at each application date were based on soil nitrate quick tests. In total, 56% less fertilizer was applied to the CM treatment. However, the CM treatment also had 4% lower commercial yield than the standard treatment. At the first fertilization, we reduced the fertilizer application in the CM treatment by 44 lbs N/A because of adequate levels of residual soil nitrate in the first foot of soil; subsequent to this application, the soil nitrate levels between CM and the standard treatment did not differ, and we do not think that reducing the N application in this fertilizer event affected the yield of broccoli. On the next application date, at 49 days after planting, 44 lbs N/A were applied in the standard treatment, but no N was applied to the CM treatment because the soil nitrate levels in the 2^nd foot were at 25 ppm nitrate-N. We suspect that the observed yield reduction in the CM treatment may have been because the level of nitrate in the upper foot was too low and the root system was not yet sufficiently developed to take up the N from the 2^nd foot. In 2015, we observed that 28% of the roots were in the 2^nd foot of soil at 52 days after planting (Figure 2), and this level of root activity may not be adequate to utilize deep soil nitrate to support rapid top growth of the crop.

Broccoli residue as a source of N: At harvest, 25-30% of broccoli biomass leaves the field as the harvested product. Given that summer-grown broccoli takes up >330 lbs N/acre, a substantial quantity of nitrogen remains in the field after harvest. For broccoli residue with high N content (5% N), more than 60% of the N is mineralized 4 weeks after incorporation into the soil. Broccoli residues with lower N content (3.75% N) will release about 40% of the N by 4 weeks after incorporation into the soil.   (Figures 3a and 3b). Figure 4 shows the results of N mineralization of broccoli residues in the field. Soil nitrate levels were evaluated in plots with and without broccoli residue. Soil nitrate-N levels peaked between 22 to 50 ppm. After the peaks, the levels declined due to rain events that leached the nitrate deeper in the soil profile (Figure 5). Soil nitrate concentrations peaked in the 2^nd foot of soil after the decline in concentration in the 1^st foot. Later, nitrate was leached from the 2^nd foot to deeper in the soil profile. The levels of nitrate mineralized from broccoli residues are substantial and can potentially substitute for a substantial amount of the fertilizer N of a subsequent crop if it is not leached due to a major rain or irrigation event.

Figure 1. Depth of the deepest broccoli roots over the course of the crop cycle (0.0 = planting to 1.0 harvest)

Figure 2. Percent of roots at various depths in the soil on four evaluation dates after germination (mean of three fields)

Figure 3a. Percent of N mineralized: broccoli residue 3.75% N (initial N in soil = 108 ppm)three temperatures.

Figure 3b. Percent of N mineralized: broccoli residue 5.04% N (initial N in soil = 125 ppm three temperatures.

Figure 4. Levels of mineral nitrogen in the soil with and without Broccoli crop residue, fall through spring 2014-2015.

Figure 5. Response of levels of residual soil nitrate from broccoli residue to rainfall events

Table 1. Analysis of total N available from residual soil N and fertilizer, broccoli N uptake and its impact on N utilization and nitrate leaching

1 – total N in the top three feet of soil; 2 – calculated by subtracting total residual soil N and crop uptake plus total mineral N from total available mineral N (also includes an estimate of N mineralized from soil organic matter of 30 lbs N/A as part of the total available mineral N)

Table 2. Applied fertilizer and biomass of CropManage vs standard and unfertilized treatments.

In the past week there have been a large number of lettuce samples from diverse parts of Monterey County submitted to our office with damage to the roots that looks like wind whip. Wind whip is an issue that we normally see to a small extent each year at certain times of the season in the Salinas Valley. However, the level of damage that growers and PCAs are reporting right now is higher than we normally see (e.g. 10 to 30%).

Wind whip is usually a short lived phenomenon in lettuce production fields. It typically is initiated during thinning when divots are cut to remove unwanted plants. If the divot is too deep or too close to the lettuce plant, it can lean excessively to one side or the other or actually twist. In either case the hypocotyl between the soil level and the base of the plant can get pinched which can be either lethal or have sub lethal effects on the plant. Given the high winds that we experience in the Salinas Valley, it is quite amazing that we do not see more damage to lettuce fields from wind whip.

According to the King City CIMIS weather station there were particularly strong winds on January 30 and February 9, 11 and 24. Many of the fields that were affected were thinned around or before these dates. It is interesting that most of the samples that we received came into the office between February 25 and March 1 indicating when the symptoms became obvious to the growers and PCAs. It appears that symptoms developed slower on this “winter” grown lettuce than it normally develops during summer months.

The wind whip samples that we saw in the field looked a bit like fertilizer burn or ammonium toxicity due to stunting, yellowing and wilting of the plants (Photos 1 – 5) (see Table 1). Upon close examination of the plants, they all had roots that were impaired by the constriction at the crown of the plant. The herbicide Goal can cause damage to the crown of the plant (Photo 6), but there was no Goal used on any of the beds with the wind whip issue that we have received to date.

Wind whip also shares some symptoms in common with soilborne diseases of lettuce. The closest match is probably Fusarium wilt, which likewise causes lettuce to be stunted, chlorotic, and wilted in late stages of disease development. Both wind whip and Fusarium wilt cause the inner core of crown/taproot to become discolored. However, Fusarium wilt in our county is a mid- to late-summer disease; it is very unusual to have Fusarium wilt of lettuce in February. Early development of Sclerotinia lettuce drop can also cause lettuce to fall behind in development, turn yellow, and collapse. Lettuce drop, however, always causes the crown to be soft and decayed, with white mycelium and black sclerotia present. See Table 1 for a summary and comparison of these symptoms.

Table 1. Comparison of wind whip and soilborne disease symptoms

Wind whip symptoms varied from slight (Photo 7) to increasingly severe to severe (Photos 8-11). The functioning of the roots is impaired by the constriction and damage to the phloem. As a result the roots often are stunted and have discolored cores indicating stress (Photos 12-13). As the roots lose their ability to function, the plant cannot grow normally and is stunted.

Wind whip appeared to be worse on east-west fields, and appeared to be worse on romaine vs head lettuce. One question that we were not able to settle was if wind whip was less severe on auto thinned fields due to the lack of disturbance of the beds in the thinning process.

The widespread occurrence of wind whip in the last few weeks appears to be literally a perfect storm of high winds and susceptible lettuce. Hopefully the rest of the year will not have as severe of conditions.

1. Distribution in field	2. Stunted and yellowed plant
3. Wilted plant	4. Fertilizer burn (1-2 deep in soil)
5. Ammonium toxicity	6. Goal damage on lettuce crown
7. Normal plant	8. Moderate wind whip
9. Slight wind whip	10. Moderate wind whip
11. Severe wind whip	12. Discoloration of root from wind whip
13. Discoloration of root core